Hi and welcome to TCS!
Lily is such a pretty girl!
I have a 2-year-old male cat, Peter, who has a heart mumur that was first detected when he was 9-months-old. He's doing great so far and is very active, playful and sweet boy.
I'm wondering if your vet recommended any additional testing, such an xray or ultrasound. Some cats, like people, can live for years with heart murmurs, with no problem, but there's a chance the murmur could be a sign of heart disease. Xrays and a cardiac ultrasound are the best ways to determine this. I went this route with Peter and we learned he has very mild HCM (hypertrophic cardiomyopathy - which gradually causes the walls of the heart to enlage and thicken.) There's no cure, but daily medication can slow or even stop the progress of these disease.
I'm only mentioning this because I wouldn't have known Peter had HCM if my vet hadn't pushed the additional testing. He hasn't shown any other symptoms (lethargy, shortness of breath, loss of appetite) and is otherwise healthy.
It might be worthwhile discussing this with your vet, if you haven't already. I'd be glad to answer any questions you might have, so feel free to PM me. Sending lots of vibes to you and Lily!
Also, here's an online support group for people whose cats have heart conditions. I've found it very helpful:http://tech.groups.yahoo.com/group/feline-heart/